The present invention generally relates to structures responsive to changes in environmental conditions, such as heat, humidity, etc., and processes for producing such structures. More particularly, this invention relates to bimorphic structures and methods of forming a bimorphic structure having a well-defined contact area and gap relative to a fixed contact with which the contact area makes contact in response to changes in an environmental condition being monitored.
Bimetallic beams and other bimorphic structures exhibit a bimorphic effect that renders such structures capable of monitoring temperature or another environmental condition. As understood in the art, the term “bimorphic effect” is used to denote the bending of a composite structure resulting in changes in the environment of the composite structure, such as changes in temperature that cause the composite structure to deflect as a result of differences in the coefficients of thermal expansion (CTE) of the materials used to form the structure.
Bimorphic beam structures have been adapted for use in a variety of systems and produced by various processes, a non-limiting example of which is commonly-assigned U.S. patent application Ser. No. 11/383,200 to Yazdi, the contents of which are incorporated herein by reference. Yazdi discloses a sensor that makes use of a large number of bimorphic structures. The bimorphic structures are MEMS (micro-electro-mechanical systems) that operate as switches for the purpose of measuring cumulative heat, humidity, etc., to enable continuous monitoring of storage and shipping conditions for various items. Yazdi's sensor finds use in a variety of applications, including supply-chain management of perishable goods, pharmaceuticals, chemicals, and fresh agriculture products.
Bimorph beams have been fabricated using micromachining technology and are typically formed in the following manner. A conducting contact pad is first formed in a substrate (such as silicon) that has an insulating layer. A sacrificial layer is deposited on the substrate and over the contact pad, and is then patterned and etched to define a first opening to the contact pad and a second opening in which an anchor for the beam will be subsequently formed. The materials of the bimorphic (e.g., bimetallic) structure are deposited on the sacrificial layer using thin film processes, after which the beams are defined by using photolithography and etching techniques. The sacrificial layer is then removed using an etchant that is selective to the materials used to form the beam. If the bimorphic structure is responsive to temperature, individual beams can be fabricated to deflect so that, at a certain temperature and beam length, the beam will contact the contact pad on the substrate. This contact can be detected and used to measure or otherwise monitor temperature. The contact of the beam with the contact pad can vary because line or point contact may occur, which can introduce unreliabilities when used to detect the closure of the contact. Consequently, it would be desirable if the contact area of a bimorph beam with a fixed pad could be rendered more well defined, repeatable and reliable.